Automatic transmissions are composed of a plurality of gear trains. For the purpose of shifting speed ranges, these gear trains establish a plurality of power transmission paths, and these transmission paths are selected through some engaging elements such as clutches and brakes, which are actuated hydraulically. While the transmission is being shifted, an engaging element establishing the power transmission path for an off-going speed range (off-going engaging element) is released, and another element (on-coming engaging element) is engaged to establish the power transmission path for an on-coming speed range.
In order to execute such shifts smoothly without any lag, the engagement and disengagement of the engaging elements are precisely timed, and forces applied to these engaging elements for engagement and disengagement are properly controlled. For example, Japanese Laid-Open Patent Publication No. 62(1987)-246653 discloses a following method. After a shift command is generated, while the engagement actuation pressure of the off-going engaging element is lowered to allow some slipping in the off-going engaging element, the on-coming engaging element is cleared of the void stroke. Then, the off-going engaging element is released while the on-coming engaging element is gradually brought into engagement. Thereafter, the actuation pressure of the on-coming engaging element is gradually increased.
Also, Japanese Patent Publication No. H6(1994)-8665 discloses a shift control method for carrying out a shift-down operation in power-on condition. In this control method, the engagement actuation pressure of the off-going engaging element is gradually lowered while the rotational speed of the turbine of the torque converter increases in response. Then, the engagement actuation pressure of the off-going engaging element is increased again to adjust this rotational speed to a target rotational speed suitable for the on-coming engaging element to engage, and the torque transmission path is switched from the off-going speed range to the on-coming speed range.
However, in this shift-down operation, if the off-going engaging element is released too quickly for the on-coming engaging element to follow up in engagement after the release of the off-going engaging element, a problem of shift shock or engine racing is experienced during the engaging of the on-coming engaging element at the end of the shift operation. On other hand, if the off-going engaging element is released too slowly, a problem of overheat or wear is experienced in the off-going engaging element because of a resultant, excessive time for slippage. Moreover, if the timing for engaging the on-coming engaging element is not controlled precisely in relation with the release of the off-going engaging element, a problem of shift shock or shift delay is likely to happen.